Firefox doesn't match the theme (light/dark) of Gnome 42

[Yochanan]

Night Theme Switcher can change both the system theme and legacy application theme. In Night Theme Switcher settings, make sure Themes > Switch GTK Theme is enabled with a corresponding theme for the Day and Night variant.

Another option:

@Yochanan

To automatically switch the system theme and legacy application theme when toggling between the Light and Dark style in Settings, install the Legacy (GTK3) Theme Scheme Auto Switcher extension (gnome-shell-extension-legacy-theme-auto-switcher).

Firefox will use the legacy application theme as it’s a Gtk 3 application.